The present invention relates to semiconductor laser diodes, and particularly to laser diodes with wavelength stabilized output.
Optical communications systems use fiber amplifiers that are optically pumped. Two such types of fiber amplifier are the erbium-doped fiber amplifier and the fiber Raman amplifier. The spectrum and output power of the laser should be stable, in order to maintain constant gain.
One problem, however, is that the pump power available from single transverse mode semiconductors is typically less than about 300 mW, and so obtaining higher pump powers requires combining the outputs of multiple lasers, which adds expense and complexity. Another problem is that conventional Fabry Perot or DFB lasers suffer from large fluctuations in output power and output spectrum that arise from changes in current or temperature. These fluctuations in output power and spectrum result in changes in the gain of the EDFA, and the amplitude of the communications signal becomes unstable. Stabilization of the pump wavelength is required for achieving both gain flatness and for obtaining higher pump power by wavelength division multiplexing (WDM) of several pump laser.
Another problem is the presence of nonlinear parasitic processes, such as stimulated Brillouin scattering (SBS), when the power level of the narrow linewidth light in the fiber becomes too high. The SBS threshold in a fiber for light from a DFB single-mode semiconductor laser, typically having a bandwidth of around 20 MHz, is generally in the range of 5 mW to 10 mW. These low SBS thresholds effectively cap the pump power deliverable from available sources, including single mode lasers, master oscillator/power amplifier (MOPA) systems, and even multimode lasers having a spectral intensity greater than about 10 mW per 20 MHz.
Therefore, there is a need for a high power laser for pumping fiber systems, such as amplifiers and lasers, that can deliver higher power than conventional single mode lasers, whose output is more stable in power and spectral content, and that overcomes the power limit set by parasitic nonlinear processes.
A semiconductor light source uses a flared gain section to provide increased power that is couplable into a single mode fiber. A frequency selective reflector, such as a fiber Bragg grating, may be used to provide feedback of the output from the semiconductor light source to induce coherence collapse of the output from the semiconductor light source. The light source may be used to provide high optical power levels within fibers for various fiber-based applications such as pumping fiber amplifiers or fiber lasers. A single such light source may be used in conjunction with multiple fiber applications. Multiple such light sources may be used together.
In one embodiment of the invention, a pump laser includes a semiconductor gain element having a first end and an output end, an optical gain region being disposed between the first end and the output end, a width of the optical gain region being greater at the output end than at the first end. The pump laser further includes an optical fiber having an input end and including a frequency-selective reflector to provide reflectance at a wavelength of light amplified in the optical gain region of the semiconductor gain element, and a light coupling system disposed to optically couple light from the output end of the gain element into the input end of the optical fiber.
Another embodiment of the invention is a semiconductor pump source having a semiconductor gain element with a rear facet and having a flared gain section, a wide end of the flared gain section being coupled to a front output facet of the gain element. An optical fiber has a first end, and a refractive index grating is formed in the optical fiber to provide reflectivity at a wavelength of light amplified in the semiconductor gain element. A light coupling system is disposed to optically couple light from the front facet of the gain element into the first end of the optical fiber. The refractive index grating has a reflectivity, and is disposed at a distance from the semiconductor gain element, both the reflectivity and the distance being selected for coherence collapse operation of the pump source.
Another embodiment of the invention is an optical fiber system that has a first pump laser and a first excitable fiber medium coupled to receive pump light from the first pump laser. The first pump laser includes a first semiconductor gain element having a first end and an output end, an optical gain region being disposed between the first end and the output end, a width of the optical gain region being greater at the output end than at the first end. A first optical fiber has an input end and includes a frequency-selective reflector to provide reflectance at a wavelength of light amplified in the optical gain region of the semiconductor gain element. A first lens system is disposed to optically couple light from the output end of the gain element into the input end of the first optical fiber.
In another embodiment of the invention, a semiconductor laser device includes a semiconductor gain element having a first end and an output end, an optical gain region being disposed between the first end and the output end, a width of the optical gain region being greater at the output end than at the first end. An optical fiber has an input end and includes a wavelength-selective reflector to provide reflectance at a wavelength of light amplified in the optical gain region of the semiconductor gain element. A light coupling system is disposed to optically couple light from the output end of the gain element into the input end of the optical fiber. A mode selective region is formed in the device for preferentially selecting a lowest order transverse mode.
In another embodiment of the invention, a semiconductor gain element has a first end and an output end, and an optical gain region is disposed between the first end and the output end. A width of the optical gain region is greater at the output end than at the first end. A light coupling system is disposed to optically couple light from the output end of the gain element into the input end of an optical fiber. An optical feedback element is disposed to feed light from the output end of the semiconductor gain element back into the semiconductor gain element. The optical feedback element preferentially feeds light back to the semiconductor gain element so that a coupling efficiency for light output from the output end and coupled into guided modes of the optical fiber is greater than 30%.
In another embodiment of the invention, a laser device includes a semiconductor amplifying means for amplifying light, having a first end and an output end wider than the first end. The laser device also includes an optical feedback means for reflecting light received from the output end of the semiconductor amplifying means, the optically reflecting means being reflective over a pre-selected wavelength range. A light coupling means couples light between the output end of the semiconductor amplifying means and the optically reflecting means. Light output from the semiconductor amplifying means is coherence collapsed.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. Other objects and attainments, together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.